This invention relates to the heating of a metallic melt, and more particularly, to an apparatus and method for heating a metallic melt with plasma torches in which contamination of the metallic melt is reduced.
Apparatus for generating high voltage plasma arcs and methods for heating metallic material with such arcs have been known for some time. Typically, prior apparatus of this type include a hearth for receiving the metallic material, and a plasma arc torch mounted above the hearth for heating the metallic material. The arc generating circuit comprises a power supply for the plasma torch and an external electrode mounted in the hearth, such that when the circuit is in operation, current passes from the power supply to the plasma torch where an arc is generated that extends onto the metallic melt. The current from the arc then passes through the melt to the external electrode which is connected with the power supply to complete the circuit. Such apparatus and methods generate a large amount of heat in the metallic melt and have the advantage of quickly and efficiently heating and melting it. U.S. Pat. Nos. 3,749,803 and 3,779,182 are generally related to plasma arc heating systems of this type.
The above described apparatus and methods have a significant drawback in some applications, in that the molten material generated in the furnace is often contaminated by the external electrode. The external electrode is connected to the power supply at one of its poles and contacts the metallic melt at its other pole, remaining in contact with the metallic melt throughout the heating operation. Such electrodes are generally composed of graphite, i.e. carbon, and when the circuit is in operation, a large electrical current passes through the electrode, and the current and heat tends to break down the carbon in the electrode with the result that the electrode deteriorates. As the circuit is maintained and the melt turns molten, the electrode continues to deteriorate and the free carbon resulting from the deterioration contaminates the molten material. Such contamination reduces both the value and utility of the molten material and limits the number of applications for which plasma torch furnaces may be used.
It is an object of the present invention to overcome the disadvantages pointed out above in connection with known apparatus and methods of heating a metallic melt with plasma torches and to provide an apparatus and method for heating a metallic melt with plasma torches in which the contamination from the external electrode is effectively avoided. More particularly, it is an object of this invention to provide an apparatus and method of heating a metallic melt in which the external electrode is withdrawn from contact with the metallic melt and removed from the arc generating circuit in order that the current through the electrode can be discontinued so that the deterioration of the electrode is minimized and the contamination of the melt is reduced. The external electrode can be withdrawn from its contact with the metallic melt shortly after the requisite circuit is established with the result that the opportunity for contaminating deterioration of the electrode is substantially eliminated. Additionally, the reduced deterioration of the external electrode minimizes the frequency with which it must be replaced.
The apparatus and method of the present invention in its preferred embodiment is comprised of a furnace chamber having a hearth for receiving a metallic melt, and a pair of plasma torches each having a rear electrode. Both plasma torches are positioned to direct an arc onto the metallic melt in the hearth. An external electrode is mounted on the furnace chamber and is movable between a position in contact with the metallic melt and a position withdrawn from contact with the metallic melt. A first direct current power supply is mounted externally of the furnace chamber and is connected between one of the plasma torches and the external electrode with its anode connected to the rear electrode of such plasma torch and its cathode connected to the external electrode. A second direct current power supply is mounted externally of the furnace chamber and is connected between the other plasma torch and the external electrode with its cathode connected to the rear electrode of such plasma torch and its anode connected to the external electrode.
During the operation of the present invention, two series circuits of opposite polarity are established, each circuit containing one of the plasma torches, one of the power supplies, the metallic melt and the external electrode. The external electrode is then withdrawn from both circuits, thereby establishing a single circuit containing both plasma torches, both power supplies and the metallic melt.
Other advantages and a fuller understanding of the invention will be had from the following detailed description when taken in conjunction with the accompanying drawings.